Let grdimage accept illum=true

[joa-quim]

No description provided.

[codecov-io]

Codecov Report

Merging #294 into master will decrease coverage by 0.05%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
- Coverage    93.9%   93.84%   -0.06%     
==========================================
  Files          80       80              
  Lines        3658     3656       -2     
==========================================
- Hits         3435     3431       -4     
- Misses        223      225       +2

Impacted Files	Coverage Δ
src/gmtlogo.jl	100% <ø> (ø)	⬆️
src/common_options.jl	95.89% <0%> (-0.16%)	⬇️
src/grdimage.jl	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13fb449...cc6ea2f. Read the comment docs.